The thought of preparing a meal for several dozen people can be rather intimidating for novices. But the basics of making soup are the same no matter how big the batch, you just have to increase the proportions. The most important thing is to remember a few simple principles: stay calm, do the math and allow lots of extra time.
Things You'll Need
Large soup pot
Three sticks butter, or 3/4 cup oil
3 pounds onions, diced
3 pounds carrots, diced
2 pounds celery, diced
Six to 10 cloves garlic, minced
18 quarts chicken or vegetable broth
Five bay leaves
3 to 4 pounds canned tomatoes, drained and coarsely chopped
3 tbsp. each of salt and pepper
Additional fresh or frozen vegetables such as green beans, okra, rutabagas, parsnips or peas (optional)
Calculate how much soup you'll need. If the soup will be part of a larger meal, an eight ounce portion is fine. If soup is to be the main course, you should allow for 12 ounces per portion. This gives a total of 17 1/2 quarts for the smaller portion, or 26 1/4 quarts for the larger portion.
Scale your favorite vegetable soup recipe to reflect the necessary number of portions. To do this, divide the quantity needed by the quantity your recipe makes. For example, if your recipe makes two quarts, divide 17.5 by two and you'll get 8.75.
Multiply every ingredient in your recipe by the corresponding number, 8.75 in this example, and the result will be a batch in the correct size. Use common sense, and don't work too hard at figuring out now much.275 of an onion is. Alternately, consult your local library or the Internet for a recipe that is already large enough for 70.
Prepare as much of the recipe as you can, ahead of time. Peel and dice vegetables, prepare or purchase the broth, and try to buy or borrow a pot large enough to make the soup in a single batch. Supermarkets often have packaged diced vegetables, which are a great time saver. So are frozen vegetables.
Making the Soup
Warm the pot over a moderate burner. Add the butter or oil, onions, celery and carrots. Stir until the onions are translucent and aromatic - don't let them brown. Add the garlic and stir for one more minute, until the garlic smell begins to mellow.
Add the broth, bay leaves and any slow-cooking vegetables. Bring the pot up to a simmer. Depending on your stove, this may take as much as an hour. If the pot will fit over two of your stove's burners, that will help speed the process.
Time the pot once it comes to a simmer, and add any remaining fresh vegetables after 45 minutes of simmering time. When all of the fresh vegetables are tender, add the tomatoes and frozen vegetables, if you are using any. Add the recommended amount of salt and pepper, as well as any fresh herbs.
Taste the soup once the frozen vegetables are tender. Adjust the seasonings as necessary, and serve hot.
Increase the ingredients by half to make the larger quantity of soup. If you wish, you can replace part of the broth with the juice drained from the tomatoes.
Do not add the tomatoes or tomato juice until the other vegetables are cooked, because the acidity can change the texture of the other vegetables.